1. Field of the Invention
The present invention relates generally to optical amplifiers, and more specifically, to an apparatus and method of amplifying optical signals at different wavelengths such that the optical signals experience substantially equal gain.
2. Description of the Related Art
Commercially available erbium-doped fiber amplifiers (EDFAs) currently have gain over a large optical bandwidth (up to about 50 nm in silica-based fibers). Over this bandwidth, the gain may depend strongly on the wavelength of the input signal. For many applications, especially long-haul fiber communications, however, it is highly desirable to operate with wavelength-independent gain. To take advantage of the enormous fiber bandwidth, signals with different wavelengths falling within the gain bandwidth of the EDFA are carried simultaneously on the same fiber bus. If these signals experience different gains, they will have different powers at the output of the bus. This imbalance becomes more acute as the signals pass through each successive EDFA, and can be significant for very long haul distances. For example, at the output end of a transoceanic bus involving dozens of EDFAs, signals experiencing a lower gain per EDFA might carry tens of dB lower power than signals experiencing higher gain. For digital systems, the difference in signal power levels must not exceed 7 dB, or the lower power signals will be too noisy to be useful. Flattening the gain of the EDFAs would eliminate this problem and produce amplifiers that can support a considerable optical bandwidth and thus a higher data rate. Because the projected world demand for EDFAs is extremely large, developing methods to flatten the gain of amplifiers while retaining high power efficiency has been and continues to be very important.
Several methods have been developed over the past few years to produce EDFAs with as flat a gain over as broad a spectral region as possible. A first method is to adjust the parameters of both the fiber (erbium concentration, index profile, nature and concentration of the core codopants) and the pump (power and wavelength). This method can produce gains that are relatively flat (xc2x11-2 dB), but only over a spectral region having a spectral width on the order of 10 nm, which is too limited for most applications.
Another method is to replace each EDFA by a combination of two concatenated fiber amplifiers, in which the two amplifiers have different respective gain dependencies on signal wavelength. These dependencies are designed to compensate each other and produce a fiber amplifier combination having gain that is nearly wavelength independent over a wide spectral region. (See, for example, M. Yamada, M. Shimizu, Y. Ohishi, M. Horigushi, S. Sudo, and A. Shimizu, xe2x80x9cFlattening the Gain Spectrum of an Erbium-Doped Fibre Amplifier by Connecting an Er3+-Doped SiO2xe2x80x94Al2O3 Fibre and an Er3+-doped Multicomponent Fibre,xe2x80x9d Electron. Lett., vol. 30, no. 21, pp. 1762-1765, October 1994.) This has been accomplished by using fibers having different hosts (e.g., a fluoride and a silica fiber) and with an EDFA combined with a Raman fiber amplifier.
A third gain equalization method is to add a filter at the signal output end of the Er-doped fiber, in which the filter introduces loss at those portions of the spectrum exhibiting higher gain. This approach has been demonstrated using filters made from a standard blazed fiber grating. (See, for example, R. Kashyap et al., xe2x80x9cWideband Gain Flattened Erbium Fibre Amplifier Using a Photosensitive Fibre Blazed Grating,xe2x80x9d Electron. Lett., vol. 29, pp. 154-156, 1993.) This approach has also been demonstrated using filters from long-period fiber gratings. (See, for example, A. M. Vengsarkar et al., xe2x80x9cLong-Period Fiber-Grating-Based Gain Equalizers,xe2x80x9d Opt. Lett., vol. 21, pp. 336-338, March 1996.)
A fourth method is gain clamping. With this approach, the EDFA is placed in an optical resonator where it is forced to lase. In a laser cavity above threshold, at a given laser wavelength, the round-trip gain is equal to the round-trip loss, irrespective of the pump power. (See, for example, Y. Zhao, J. Bryce, and R. Minasian, xe2x80x9cGain Clamped Erbium-doped Fiber Amplifiersxe2x80x94Modeling and Experiment,xe2x80x9d IEEE J. of Selected Topics in Quant. Electron., vol. 3, no. 4, pp. 1008-1011, August 1997.)
In the gain clamping experiment of Zhao et al., the resonator was made of two fiber gratings that exhibit high reflectivity only over a very narrow bandwidth around a particular wavelength xcex0 (and little reflectivity at other wavelengths within the gain spectrum of the erbium-doped fiber), so that lasing took place only at this wavelength xcex0. The selection of xcex0 greatly affects the spectral shape of the EDFA gain. By selecting the proper laser wavelength xcex0 (1508 nm in their experiment), the gain spectrum can be relatively flat over a fairly broad region. Furthermore, the gain at xcex0 is clamped to the value of the cavity loss at this wavelength for any pump power above threshold. If the gain is homogeneously broadened, the gain at other wavelengths also remains independent of pump power (assuming the pump power is above threshold).
Another way to flatten the gain of a gain-clamped EDFA is to rely on the inhomogeneous broadening of the laser ions. Although reference is made herein to xe2x80x9claser ions,xe2x80x9d the discussion can be applied to any particle that produces lasing via stimulated emission, such as ions, atoms, and molecules. In a laser medium that is purely homogeneously broadened, all the ions exhibit the same absorption and emission spectra. When such a material is pumped below laser threshold, the round-trip gain is lower than the laser resonator round-trip loss at all frequencies across the laser gain spectrum, as illustrated in FIG. 1A, where it was assumed without loss of generality that the round-trip loss is frequency-independent across the gain spectral region. When pumped just above threshold, it begins to oscillate at the wavelength xcex1, that satisfies the condition gain=loss (see FIG. 1B). As the pump power is increased further (FIG. 1C), the condition gain=loss continues to be satisfied at xcex1, i.e., the gain at xcex1 remains constant. This can be understood from a physical point of view as follows. When the pump power is increased, the population inversion increases, which produces more intense laser emission. While circulating through the fiber, this larger laser signal depletes the population inversion via stimulated emission just enough so that the gain remains equal to the loss. Further, since the broadening is homogeneous, all ions contribute equally to the gain at xcex1, and therefore, the gain spectrum does not change. As a corollary, the laser wavelength (xcex1) and the laser linewidth also remain the same (see FIG. 1C), i.e., they are independent of pump power. This is the basis for the gain stabilization method mentioned earlier.
In a laser medium that is strongly inhomogeneously broadened, on the other hand, not all ions exhibit the same absorption and emission spectra. One reason for this behavior is that not all physical sites where the laser ions reside are identical. For example, a laser ion can reside next to a silicon ion, an oxygen ion, or an aluminum ion in the case of an aluminum-doped silica-based host. Laser ions residing at identical sites (e.g., all the laser ions next to a Si ion) will exhibit the same absorption and emission spectra, i.e., they will behave homogeneously with respect to each other. On the other hand, laser ions residing at different sites, e.g., one residing next to a Si ion and another laser ion residing next to an Al ion, will exhibit different absorption and emission spectra, i.e., they will behave inhomogeneously with respect to each other. In the case of inhomogeneous broadening, the laser medium can thus be thought of as a collection of subsets of laser ions. Ions within a given subset behave homogeneously, while ions in different subsets behave inhomogeneously.
When an inhomogeneously broadened material is pumped below laser threshold, the round-trip gain is lower than the laser resonator round-trip loss at all frequencies across the laser gain spectrum, as illustrated in FIG. 2A, assuming a round-trip loss that is frequency-independent across the gain spectral region. When this material is pumped just above threshold, it will first oscillate at the wavelength xcex1 that satisfies the condition gain=loss (see FIG. 2B and compare with FIG. 2A, which is the below threshold case). This laser emission predominantly involves the ion subsets exhibiting substantial gain at xcex1. As the pump power is increased, laser emission at other wavelengths will begin to appear, although the condition gain=loss continues to be satisfied at xcex1, as illustrated in FIG. 2C. Once again, the laser medium meets this condition by producing just enough laser power to reduce the population inversion by precisely the amount that the population inversion had increased due to the increase in pump power. The gain at xcex1 is thus xe2x80x9cclampedxe2x80x9d at the value of the loss. However, since the broadening is inhomogeneous, the gain available from the other ion subsets peaking at wavelengths other than xcex1 is not nearly as strongly depleted by the laser power at xcex1. Consequently, as the pump power is increased, the gain at these other wavelengths (for example, wavelength xcex2) increases until it reaches the level of the loss at that wavelength, and the medium begins to lase at xcex2. At this point, the gain is clamped at both xcex1 and xcex2. In general, because the gain curve is bell-shaped, xcex2 is very close to xcex1 (FIG. 2C). As still more pump power enters the fiber (FIG. 2D), more and more wavelengths begin to lase. In practice, each of these discrete laser lines actually has a finite linewidth. Thus, if these discrete lines are close enough to each other, they merge with each other and the net effect of this increase in the number of lasing lines is that the laser linewidth broadens. In short, an inhomogeneously broadened laser medium tends to produce laser emission that broadens with increasing pump power. The laser linewidth can in principle increase in this fashion until it reaches the gain linewidth.
In general, the laser transitions of triply ionized rare earth elements like Er3+ are broadened by both homogeneous and inhomogeneous processes. Homogeneous mechanisms broaden the linewidth of the transitions between the Stark sublevels of the erbium ions in the same manner for all Er ions in the host. On the other hand, some inhomogeneous mechanisms produce changes in the distribution of the Stark sublevels which are not the same for all ions, but which depend on the ion subset.
At room temperature, the 1.55 xcexcm transition in Er-doped silica is predominantly homogeneously broadened. However, by cooling the material to cryogenic temperatures, it is possible to reduce the homogeneous broadening and produce a laser that oscillates over a relatively broad spectral range of constant gain (equal to the resonator loss). This effect has been used to produce flat gain in an EDFA operated at 77xc2x0 K. (See, for example, V. L. da Silva, V. Silberberg, J. S. Wang, E. L. Goldstein, and M. J. Andrejco, xe2x80x9cAutomatic gain flattening in optical fiber amplifiers via clamping of inhomogeneous gain,xe2x80x9d IEEE Phot. Tech. Lett., vol.5, no. 4, pp. 412-14, April 1993.) However, this approach is in general impractical because of the apparatus required to cool the fiber.
A preferred embodiment of the present invention utilizes the inhomogeneous broadening of the 1.55 xcexcm transition of erbium to produce flat gain in an erbium-doped fiber amplifier without the need to cool the fiber to cryogenic temperatures. Gain broadening can be stimulated by pumping the fiber on the edge of the absorption band of the erbium ions, in contrast to existing erbium doped fiber amplifiers (EDFAs), which are pumped at or near the center of the 980-nm absorption band. Alternatively, the erbium doped fiber can be pumped at multiple wavelengths simultaneously to excite a large number of subsets of erbium ions, producing gain over the broadest possible spectral region. For example, for pumping on the 4I15/2xe2x86x924I11/2 transition, the pump wavelengths can be distributed, uniformly or otherwise, between around 970 nm and around 990 nm to cover a substantial portion of the absorption spectrum. The ideal spectral extent of the pumping spectrum depends on the absorption spectrum of the particular erbium-doped fiber used, which itself depends on the codopants present in the fiber""s core region.
One preferred embodiment of the invention is an optical amplifier that includes an optical resonator for producing clamped gain, in which the resonator includes a gain medium that has an absorption profile and a gain profile, with the gain profile being characterized at least in part by inhomogeneous broadening. The optical amplifier further includes an optical pump source for pumping the gain medium at at least one wavelength in a tail of an absorption transition of the gain medium to utilize the inhomogeneous broadening to flatten the gain. In one preferred embodiment, the optical resonator is a ring resonator, and the gain medium includes a doped fiber.
Yet another preferred embodiment of the invention is an optical amplifier that includes an optical resonator for producing clamped gain, in which the resonator includes a gain medium having an absorption profile and a gain profile, with the gain profile being characterized at least in part by inhomogeneous broadening. This embodiment further comprises an optical pump source for pumping the gain medium in a tail of an absorption transition of the gain medium to utilize the inhomogeneous broadening to modify the gain, and also comprises a wavelength-dependent loss element for adjusting the loss to produce a desired gain profile.
Still another preferred embodiment of the invention is a method for producing an optical amplifier having substantially flat gain, in which the method includes introducing a pump signal into a gain medium having an absorption profile and a gain profile, in which the gain medium resides within a resonator. The gain profile is characterized at least in part by inhomogeneous broadening, and the spectral output of the pump signal is selected to pump a tail of the absorption profile to utilize the inhomogeneous broadening of the gain medium. This method further comprises injecting a plurality of optical signals of different wavelengths into the gain medium to amplify the optical signals, in which the respective wavelengths of the optical signals fall within the gain profile of the gain medium, and utilizing stimulated emission within the gain medium to clamp the gain of the gain medium over a spectral region that includes the wavelengths of the optical signals. Amplified optical signals are then extracted from the gain medium. In one preferred embodiment of this method, one or more codopants may be added to the gain medium to enhance the inhomogeneous broadening of the gain profile. In another preferred embodiment of this method, the gain may be controlled by varying loss within the resonator. In yet another preferred embodiment of this method, the gain flatness may be controlled by adjusting a wavelength dependent loss element within the resonator.